Maraging steels are carbon-free or low-carbon steels, and are obtained by subjecting steels containing Ni, Co, Mo, Ti and like elements in high proportions to solution heat treatment and then to quenching and aging treatment.
Maraging steels have characteristics including (1) good machinability attributable to formation of soft martensite in a quenched stage, (2) very high strength attributable to precipitation of intermetallic compounds, such as Ni3Mo, Fe2Mo and Ni3Ti, in martensite texture through aging treatment, and (3) high toughness and ductility in spite of its high strength.
Maraging steels have therefore been used as structural materials (e.g. engine shafts) for spacecraft and aircraft, structural materials for automobiles, materials for high-pressure vessels, materials for tools, and so on.
So far, 18Ni Maraging steels (e.g. Fe-18Ni-9Co-5Mo-0.5Ti-0.1Al) of Grade 250 ksi (1724 MP) have been used for engine shafts of aircraft. However, with the recent demand of improving air pollution by, for example, tightening control on exhaust gas emission, enhancement of efficiency has been required of aircraft also. From the viewpoint of designing engines, there have been increasing demands for high-strength materials capable of enduring high power, downsizing and weight reduction.
As to such high-strength materials, various types of steels have been put forth until now.
For example, Patent Document 1 has disclosed a ultrahigh tensile strength tough-and-hard steel containing 0.05 to 0.20 weight % of C, at most 2.0 weight % of Si, at most 3.0 weight % of Mn, 4.1 to 9.5 weight % of Ni, 2.1 to 8.0 weight % of Cr, 0.1 to 4.5 weight % of Mo which may be substituted partially or entirely with a doubling amount of W, 0.2 to 2.0 weight % of Al, and 0.3 to 3.0 weight % of Cu, with the balance being Fe and inevitable impurities.
In the document cited above, there is a description that strength of 150 kg/mm2 (1471 MPa) or higher can by achieved by multiple addition of Cu and Al to low-carbon Ni—Cr—Mo steel without significantly impairing toughness and weldability.
In addition, Patent Document 2 has disclosed a high-strength highly-fatigue-resistant steel containing about 10 to 18 weight % of Ni, about 8 to 16 weight % of Co, about 1 to 5 weight % of Mo, 0.5 to 1.3 weight % of Al, about 1 to 3 weight % of Cr, at most about 0.3 weight % of C, and less than about 0.10 weight % of Ti, with the balance being Fe and inevitable impurities, and further containing both of fine intermetallic compounds and carbides made to precipitate out.
In Table 2 of the document cited above are presented findings that such a steel has a tensile strength of 284 to 327 ksi (1959 to 2255 MPa) and an elongation of 7 to 15%.
Although maraging steels are generally high-strength materials which excel in toughness and ductility, it is known that, in a tensile strength range exceeding 2,000 MPa, it is difficult to ensure fatigue resistance as well as toughness and ductility. Thus, as for general-purpose materials, only Grade-250 ksi 18Ni maraging steels has been utilized so far.
On the other hand, steels of the type which are disclosed in Patent Document 2 are also known as high-grade materials for general-purpose use. However, in order to meet the demands, for example, for increasing the efficiency of aircraft, further increase in strength (2,300 MPa or higher) without attended by reduction in fatigue resistance as well as toughness and ductility has been required of maraging steels.
Against this backdrop, the present applicant has proposed Patent Document 3 as a maraging steel having a tensile strength of 2,300 MPa or higher, an elongation of 7% or larger and excellent fatigue characteristics. However, such a maraging steel is apt to form thin tabular AlN particles which are supposed to be inclusions affecting low-cycle fatigue characteristics. Accordingly, the maraging steel may suffer deterioration in low-cycle fatigue characteristics, and high-level stabilization of low-cycle fatigue characteristics may be difficult for it to achieve.
Patent Document 1: JP-A-S53-30916
Patent Document 2: U.S. Pat. No. 5,393,488
Patent Document 3: JP-A-2014-12887